Light Controlled Photoluminescence Relaxation in Porous Silicon
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EXPERIMENT Samples where cut from 7-24 IQ cm p-type, (100) oriented high purity wafers (Wacker Chemitronic) and prepared by anodic etching (curent density about 20 mA cm-2) in a mixture of HF:H 2 0:C2H5 OH=1: 1:2. The applied current density was tuned in a series of experiments to get maximum PL intensity for a given wafer material. The wet samples were washed in deionized water and dried for 15min in air. After drying, oxidation was performed by a 30 s dip in 65% nitric acid, followed by rinsing in water. To remove the rest of the acid and the water from the pores, drying was carried out under high vacuum conditions for several minutes. In this state, fast PL enhancement by a factor of 2 to 3 is possible by application of visible light [ 10]. However, it should be mentioned that a slow enhancement occurs even by long term storage in the dark. In any case, to get the effect as described below, this long term storage in ambient air is essential, even if the PL enhancement has been done by light. In order to carry out a comparative study, the
prepared material was cut into several pieces. ESR, FTIR and PL investigations were correlated, if possible, by taking data from the same sample, otherwise from similar samples of the same preparation set. ESR measurements were done at room temperature using a Bruker 200 D XBand spectrometer in absorption mode. In order to perform the FTIR measurements, a conventional Perkin Elmer spectrometer was used. PL data were collected by a modular optical multichannel system with the blue line (488 nm) of an Ar ion laser as an exitation source. EXPERIMENTAL RESULTS AND DISCUSSION Fig .1 demonstrates that the PL intensity reacts differently to the alternating influence of visible light and ageing in the dark and also depends on the preceding treatment steps. In the beginning, after the vacuum drying step following the HNO 3 dip, exposure to air causes a short
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Time [min] Fig. 1. Typical PL intensity development during various treatments of a chemically oxidised porous silicon layer. The regions signed with "a" refer to "light off', those signed with "b" to "light on", respectively.
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decrease of the PL followed by a stable period on a minute scale (period 1). Illumination with 15 mW/cm 2 488nm laser fight enhances the PL intensity in a short time (period 2). After removing the radiation source, the development of the PL intensity shows a further long term increase (period 3). After this 24 h ageing period, however, the effect of light on PL is reversed: "Light on" decreases the PL in a short time (period 4). In the dark at room temperature, the PL intensity recovers towards the initial value at the start of period 4 (period 5). The further steps shown in the figure demonstrate the reaction for successive "light on" and "light off' conditions and indicate the reversibility of the effect. Figure 2 shows PL spectra during a full relax
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